Birdsmouth Cut Calculator

Find the cut angles for a rafter's birdsmouth notch where it sits on the wall plate.

How the Math Works

The Birdsmouth Cut Calculator uses basic trigonometry to determine rafter angles. The plumb cut angle is calculated using the arctangent (atan) of the rise (vertical height) divided by 12 inches (horizontal run), which standardizes roof slope calculations. The seat cut angle is derived by subtracting the plumb cut angle from 90 degrees, forming a complementary angle. These calculations transform abstract roof geometry into precise angular measurements for carpentry.

Practical Applications

To apply this, input the rise measurement from your roof's pitch or rafter dimensions. The plumb cut angle determines the vertical 'nose' of the notch, while the seat cut angle defines the horizontal base. Mark these angles on your rafter using a protractor or angle finder, ensuring the birdsmouth fits snugly against the top plate of a wall. Always verify measurements with physical templates before cutting to prevent structural gaps or misalignment.

Day-to-Day Use

This calculator simplifies complex roof framing for DIY enthusiasts and professionals alike, turning intimidating angles into actionable steps. Accurate birdsmouth cuts ensure rafters sit securely on wall plates, preventing roof sagging or leaks. Whether building a shed, repairing a porch, or constructing a deck, precise angle measurements save time, reduce material waste, and guarantee a safer, more stable structure—making 'do-it-yourself' projects far more achievable.

Worked example

6/12 pitch → plumb cut 26.6°, seat cut 63.4°.

FAQ